1. Field of the Invention
The present invention relates to a method of injection molding thermoplastic resin and an injection molding apparatus, and particularly relates to a method of injection molding and an injection molding apparatus, which performs the injection molding of thermoplastic resin after a supercritical fluid is impregnated in molten resin.
2. Description of the Related Art
In recent years, studies using supercritical fluids as solvent are actively conducted. A supercritical fluid has not only zero surface tension and a diffusive property equivalent to gas but also a function as solvent because a supercritical fluid has density close to liquid. As a technology using such physical properties of the supercritical fluid, there has been proposed a novel electroless plating method for plastic using the supercritical fluid (see, for example, “Latest Applied Technology of Supercritical Fluid”, Teruo Hori, NTS Inc., pp. 250 to 255, 2004). The electroless plating method using the supercritical fluid is made to overcome problems as explained below in conventional technologies for forming an electroless plating film on plastic.
The electroless plating method is widely used as means for forming a metallic conductive film on a surface of an electronic apparatus composed of a plastic structure. Conventional electroless plating process for plastic differs slightly depending on materials or the like, but is generally constituted of processes through steps of molding resin, degreasing a molded article, etching, neutralizing and wetting, adding catalyst, activating catalyst, and electroless plating. Among the above electroless plating processes, a chromic acid solution, an alkaline metal hydroxide solution or the like is used in the etching step, but these etching solutions need post-treatment such as neutralizing and thus cause high costs. Further, in the etching step, a highly poisonous etchant is used, which is troublesome for handling. In Europe, RoHS (Restriction of the use of certain Hazardous Substances in electric and electronic equipment) Directive is established, which regulates certain hazardous chemical substances included in electric and electronic products, and material/part supply makers must assure that hexavalent chromium and the like are not included in new electric/electronic equipment to be released in the European market on and after Jul. 1, 2006. Under such a circumstance, it is an inevitable problem for conventional electroless plating processes for plastic, which has large environmental load, to be changed to alternative technologies.
In comparison to the above electroless plating processes having an etching step, in the method described in “Latest Applied Technology of Supercritical Fluid”, Teruo Hori, NTS Inc., pp. 250 to 255, 2004, it is possible to impregnate organic metal complexes in the surface of the polymer by dissolving the organic metal complexes in a supercritical carbon dioxide, and bringing the supercritical carbon dioxide into contact with various polymers. Then, the organic metal complexes impregnated in the polymer are reduced by heating, chemical reduction processing, or the like to thereby allow separation of metallic fine particles. Accordingly, the entire surface of the polymer can be subjected to electroless plating. According to this process, it is said that waste liquid treatment is not required, and plastic having favorable surface roughness can be obtained.
Furthermore, conventionally there is foam molding as an injection molding process which uses the supercritical fluid and is actually industrialized (see, for example, Japanese Patent No. 2625576). In this process, an inert gas such as N2 and CO2 can be used as a foaming agent instead of conventional chemical foaming agents, and the inert gas is mixed and kneaded in a supercritical state with molten resin. This mixing and kneading step is performed when a resin material is plasticized and measured using a screw in a plasticizing cylinder, while stirring the resin material to be plasticized and melted and a supercritical fluid of N2, CO2 or the like by the screw.